On Jan 25, 2007, at 10:50 AM, Milo Hyson wrote:
The write times of both RAID configurations are slower than the single drive (which is expected due to having to write to multiple drives). However, I wasn't expecting such a drastic reduction (about 50%). The read times, although faster, are only marginally so in per-char transfer. They're a bit better in block performance, but still not what I would expect. It would seem to me that a read spread across four drives should see more than a 45% performance increase. The highest rate recorded here is only a quarter of the PCI bus-speed, so I doubt that's a bottleneck. CPU load peaks at 50%, so I don't see that being a problem either.

Single-byte accesses are a worst-case scenario for RAID throughput; the block rates are generally more applicable to the performance you'll see for decently-written applications and many use-case scenarios. If you've got a UPS or battery-backup option for the RAID card enabled, consider turning on write-back mode rather than write- thru mode, which ought to improve write performance pretty significantly.

Still, you also ought to consider that a 3-disk RAID-5 configuration is very much not ideal from either an efficiency or performance standpoint-- you want more like 5 or 6 drives being used, in which case your performance numbers ought to increase some. This is also somewhat true of the 4-disk RAID-10 config; using 6 or all 8 drives would likely improve performance compared with striping against only two disks.

I also ran some performance tests with a stock build of PostgreSQL 8.0 to get a different angle on things.
[ ... ]
Any performance benefit of RAID in these tests is almost nonexistent. Am I doing something wrong? Am I expecting too much? Any advice that can be offered in this area would be much appreciated.

Most databases dislike any form of RAID except plain old RAID-1 mirroring, but absolutely hate RAID-5. Databases can do OK with big RAID-10 combinations, too, but ask any experienced DBA what they'd like, and they'd rather have as many RAID-1 spindles available as possible compared with any other drive arrangement.


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